The aim of this observational study was to investigate the postoperative alignment change with Oxford unicompartmental knee arthroplasties (UKA), and clarify whether femoro-tibial facet angle (FTFA) is useful for evaluating alignment correctability with UKA. This study evaluated 79 knees consecutive minimally invasive Oxford phase 3 UKAs performed between 2013 and 2014. Full-length weight-bearing radiographs of the lower limbs were obtained pre- and postoperatively to assess varus angle. Preoperative valgus stress radiography in the supine position was also performed. FTFA was measured on weight-bearing anteroposterior radiography and valgus stress radiography. The preoperative varus angle of 4.6A degrees +/- 3.1A degrees reduced to 1.7A degrees +/- 2.6A degrees postoperatively. Preoperative varus angle and postoperative varus angle change strongly correlated with the FTFA value and its change on the valgus stress radiographs, respectively (p < 0.01). Based on preoperative FTFA under valgus stress radiography, intra-articular varus corrected group (37 knees) with preoperative varus angle 2.9A degrees +/- 2.4A degrees was corrected to - 0.3A degrees +/- 2.0A degrees after UKA. However, intra-articular varus uncorrected group (42 knees) with preoperative varus angle 6.0A degrees +/- 3.0A degrees was only corrected to 3.5A degrees +/- 1.7A degrees. Thirteen knees (16.5%) were overcorrected to valgus after UKA, with a mean FTFA of - 1.2A degrees +/- 0.4A degrees under valgus stress force, which related with a postoperative valgus angle 0.8A degrees +/- 1.2A degrees. FTFA change under valgus stress force was useful for evaluating the correctability of UKA. It could reflect intra-articular varus deformity. Intra-articular varus deformity not corrected under valgus stress would result in varus after UKA. However, intra-articular deformity which could be overcorrected under valgus stress would have a tendency to valgus after Oxford UKA.